Delete dead keyed_cell module (vercel/turborepo#8761)

### Description

Looks like this was added with a callsite in vercel/turborepo#6160 , and the callsite was removed in vercel/turborepo#6172 .

### Testing Instructions

```
cargo check
```

crates/turbo-tasks/src/lib.rs

@@ -48,7 +48,6 @@ mod id;
 mod id_factory;
 mod invalidation;
 mod join_iter_ext;
-mod keyed_cell;
 #[doc(hidden)]
 pub mod macro_helpers;
 mod magic_any;
@@ -85,7 +84,6 @@ pub use invalidation::{
     DynamicEqHash, InvalidationReason, InvalidationReasonKind, InvalidationReasonSet,
 };
 pub use join_iter_ext::{JoinIterExt, TryFlatJoinIterExt, TryJoinIterExt};
-pub use keyed_cell::{global_keyed_cell, keyed_cell};
 pub use manager::{
     dynamic_call, emit, get_invalidator, mark_finished, mark_stateful, prevent_gc, run_once,
     run_once_with_reason, spawn_blocking, spawn_thread, trait_call, turbo_tasks, CurrentCellRef,

crates/turbopack-core/src/chunk/chunking.rs

@@ -141,8 +141,7 @@ async fn handle_split_group(
     })
 }
 
-/// Creates a chunk with the given `chunk_items. `key` should be unique and is
-/// used with [keyed_cell] to place the chunk items into a cell.
+/// Creates a chunk with the given `chunk_items. `key` should be unique.
 #[tracing::instrument(level = Level::TRACE, skip_all, fields(key = display(key)))]
 async fn make_chunk(
     chunk_items: Vec<ChunkItemWithInfo>,